Motor vehicles employ switching power supplies that utilize pulse width modulation to convert the stock vehicle voltage from the vehicle power supply (usually twelve volts) to whatever particular voltage levels are required to run the various electronic modules connected to the power supplies. The switching power supplies operate by switching on and off at a certain operating frequency; the output voltage from a power supply is determined by the duty cycle of the switched voltage.
Motor vehicle switching power supplies nominally operate in the frequency range from about 20 to 500 kilohertz (kHz). The frequencies at which the power supplies operate are chosen based on considerations such as efficiency of operation, heating, and the like. The frequencies are also chosen such that they do not cause harmful interference with the many electronic modules in the motor vehicle; however, the harmonics (whole-number multiples of the operating frequencies) generated by the power supplies may still cause interference with some of the electronic modules. The amplitude modulation (AM) band of automotive radios is particularly susceptible to interference from switching power supply operating frequency harmonics, because the AM frequency band extends from 520 (kHz) to 1710 kHz and the low-order harmonics generated by the switching power supplies fall into the AM band frequency range. A switching power supply operating at 200 kHz may cause interference for AM radio stations at 600 kHz (the power supply's third harmonic), 800 kHz (the fourth harmonic), and so forth. Because AM radios have a ten kHz resolution, meaning that they are calibrated to tune in steps of ten kHz, radio interference is experienced whenever a power supply harmonic is within plus or minus five kHz of the frequency to which the radio is tuned.
Accordingly, it is desirable to provide a method for AM band radio interference suppression that overcomes the interference problems attendant with prior art motor vehicles. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.